1. Introduction
This invention relates to new and improved coatings useful in electronics manufacture such as in the fabrication of printed circuit boards. More specifically, the invention relates to liquid photopolymerizable compositions which may be readily applied to a substrate and U.V. cured to form adherent coatings useful for a variety of purposes, and in one embodiment finds use as a solder mask characterized by unique adhesion to copper and other metals following wave soldering.
2. Description of the Prior Art
In increasing frequency, electronic components such as printed circuit boards are coated with organic coatings at some point during their manufacture. One such coating, used as a solder mask, is a permanent organic coating which is typically screen printed over an entire circuit pattern, leaving open points such as component insertion holes, which subsequently take solder. The use of a solder mask provides advantages of insulation of the circuit tracers; hermetic sealing; resistance to soldering and solder fluxes; aesthetics; use as a base for legend inks; and resistance to solder bridging.
To provide the necessary properties, most solder masks commercially used have been heat curable thermosetting, polymer systems. However, the industry is now turning to polymer systems that cure by exposure to ultraviolet radiation. U.V. curing provides savings in terms of energy consumption, equipment, space and time. U.V. curable coatings are essentially 100 percent solids that offer no pollution problems due to solvent emissions. They are easy to use because the ink will not dry on the screen in a room suitably equipped with yellow lighting and they provide improved thickness uniformity with concommitant improved protection over the edges of conductor lines. The price per kilogram of U.V. curable coating is generally greater than conventional heat-curing systems, but there are overall cost advantages associated with their use for the reasons described above.
Many formulations have been reported in the literature as useful as U.V. curable coatings. Most are based upon acrylated oligomers such as acrylated epoxies, polyesters, polyethers and urethanes. These systems rely upon a photoinduced polymerization mechanism for curing. A typical U.V. curing formulation comprises an oligomer or prepolymer, monomer(s), photoinitiator(s), pigment(s) and other additives. The oligomers or prepolymers are the materials described above. The monomers are typically low viscosity acrylate esters including monoacrylates, diacrylates and some triacrylates. The photoinitiator absorbs the ultraviolet energy and initiates the polymerization of the U.V. curable material. Typical classes of photoinitiators known in the art are the acyloins or acyloin ethers; polyhalogenated materials; and the commonest type which are the aryl ketones. A general description of such materials, and the method of formulating the same, are set forth by Pappas, U.V. Curing; Science and Technology. Technology Marketing Corporation, Connecticut, pages 187 through 202, 1980, incorporated herein by reference.
A patent believed to encompass a commercially available U.V. curable solder mask formulation is U.S. Pat. No. 4,064,287, incorporated herein by reference. The invention is said to reside in the use of a formulation free of solvent. The formulation comprises as essential components a photopolymerizable material containing an aryloxyalkyl acrylate oligomer; a liquid photopolymerizable monomer referred to in the patent as a diluent containing at least one terminal ethylenic group; and a free radical generating addition polymerizing initiating system activatable by actinic radiation. Numerous examples of each of the above-identified classes of essential ingredients are set forth in the patent.
Solder masks prepared in accordance with the teachings of the above patent and other solder masks available in the market suffer certain disadvantages. For example, they are typically applied to a printed circuit board by screening in a pattern designed to prevent unwanted soldering on portions of the conductor lines. For example, a simple etched printed circuit consists of copper conductors on a plastic substrate such as an epoxy G-10 laminate. After the solder mask has been applied, electronic components are mounted on the board through the holes. The wire leads then have to be connected to the copper conductors, which typically is performed by wave soldering or float soldering, during which operation hot molten solder (typically about 260.degree. C.) is applied to the wire leads and the exposed sections of the copper conductors, thereby forming a fused solder connection. During this soldering operation, the entire surface area of the printed circuit board is exposed to hot molten solder. Since the coefficient of expansion of copper metal is so different from the coefficient of expansion of the solder mask, tremendous strain is placed on the adhesion of the solder mask to the copper conductors which often leads to delamination of the solder mask from the copper conductor. In addition, the cured mask often does not have sufficient mechanical strength to withstand abrasion that might be encountered in a typical manufacturing process and the coatings are often brittle, whereby minor flexing of the board may result in fracture of the coating and loss of protection. Poor adhesion or failure to withstand abrasion can result in delamination of the solder mask and ultimate failure of a printed circuit.